RxJS: Closed Subjects

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This article looks at the unsubscribe method of Subject — and its derived classes — as it has some surprising behaviour.

Subscriptions

If you look at the signature for Observable.prototype.subscribe, you’ll see that it returns a Subscription. And if you’ve used observables, you will be familiar with calling the subscription’s unsubscribe method. However, a subscription contains more than just the unsubscribe method.

In particular, the Subscription class implements the ISubscription interface:

Where AnonymousSubscription is the same interface, but without the read-onlyclosed property.

The closed property indicates whether or not the subscription has been unsubscribed — either manually or automatically (if the observable completes or errors).

Subscribers and unsubscription

Interestingly, what’s actually returned from a call to subscribe is an instance of the Subscriber class — which extends the Subscription class.